A contactless power transfer apparatus supplies electrical power from a power transmission coil (primary coil) to a power reception coil (secondary coil) by using electromagnetic induction between the power transmission coil and the power reception coil. The contactless power transfer apparatus can be used to charge a secondary battery installed in an electric vehicle or a plug-in hybrid electric vehicle, and a demand for the contactless power transfer apparatus for charging the vehicles is expected to expand in the future.
In a case of the contactless power transfer apparatus for charging the vehicles, contactless power transfer is performed by parking an automobile with the power reception coil installed on an underside of a floor thereof, so that the power reception coil is positioned just above the power transmission coil placed on a ground. However, due to misalignment in a horizontal direction and a gap length variation in a vertical direction between the power transmission coil and the power reception coil, a coupling factor between the power transmission coil and the power reception coil might change.
In order to prevent a decrease in a power-receiving efficiency due to the misalignment and the gap length variation between the power transmission coil and the power reception coil, it is necessary to secure a large opposing area between the coils, whereby it is inevitable that a coil shape becomes larger in size.
However, by using a coil (double-sided winding coil) around which a wire 50 is wound at a central part of a rectangular core 70 as in FIG. 9A, it is possible to make the size smaller than a coil having a winding wire arranged on a single side of a core thereof (single-sided winding coil) as described in Patent Literature 1 below. At the top of FIG. 9A is the core provided with the winding wire, and at the bottom is the core alone. Both ends 71 and 72 of the core 70, around which the wire 50 is not wound, are magnetic pole portions where a magnetic flux flows in or out.
Furthermore, the present inventors have already proposed an H-shaped core 80, which allows for a reduction in size and saving of an amount of a core material, as in FIG. 9B (see Patent Literature 2). In this H-type core, the wire 50 is wound around a horizontal bar part 83 of an H shape, and vertical bars 81 and 82 parallel to each other are the magnetic pole portions.
As in FIG. 10, in a case where the power transmission coil and the power reception coil, which are double-sided winding coils, oppose each other, a main magnetic flux 67 flows out from a magnetic pole portion of a core 61 of the power transmission coil, enters a magnetic pole portion of a core 63 of the power reception coil, passes through a core portion thereof around which a wire 64 is wound, flows out from the other magnetic pole portion thereof, enters a magnetic pole portion of the core 61 of the power transmission coil, passes through the core portion thereof around which a wire 62 is wound, and reaches the other magnetic pole portion.
Note that in the contactless power transfer apparatus using the double-sided winding coils, aluminum boards 65 and 66 are arranged at the back of the coils in order to magnetically shield a leakage flux.
Furthermore, FIG. 11 is a circuit diagram of a contactless power transfer system described in Patent Literature 3 below. This circuit includes: a high frequency power source 3 having a rectifier 5, which rectifies an alternate current of a commercial power supply VAC, and an inverter 4, which generates and outputs a high frequency; a power transmission coil 1; a primary side series capacitor CS, which is connected in series between the high frequency power supply 3 and the power transmission coil 1; a power reception coil 2 opposing the power transmission coil 1; a secondary side resonant capacitor CP connected in parallel to the power reception coil 2; and a load RL.
This equivalent circuit can be illustrated as in FIG. 12, and a value CS of the primary side series capacitor and a value CP of the secondary side parallel resonant capacitor are set as the following based on a resonance condition:ω0=2πf01/ω0CP=ω0L2=xP=x0′+x21/ω0CS=xS′={(x0′·x2)/(x0′+x2)}+x1′
Here, f0 is a frequency of the high frequency power source 3, and L2 is self-inductance of the power reception coil.
In the contactless power transfer apparatus provided with the primary side series capacitor CS and the secondary side parallel resonant capacitor CP, the contactless power supply transformer (power transmission coil and power reception coil) becomes equivalent to an ideal transformer by setting CP and CS in this way, whereby designing thereof becomes easier. Furthermore, in a case where the load RL is a resistance load, a power factor of the high frequency power supply 3 always becomes one.